A basic process for the production of arylene sulfide polymers from polyhalo-substituted compounds by reaction with an alkali metal sulfide in a polar organic solvent is disclosed in U.S. Pat. No. 3,354,129. Since then a great deal of work has been done to provide methods of modifying or controlling the melt flow rate of arylene sulfide polymers thus produced. For example, the use of polyhalo-substituted aromatic compounds having more than two halogen substituents per molecule has been utilized to provide branched arylene sulfide polymers of low melt flow rate. One such process is disclosed in U.S. Pat. No. 4,116,947. In addition, the use of alkali metal carboxylates has provided a method for the production of modified arylene sulfide polymers of higher molecular weight and a correspondingly lower melt flow rate than that obtained in a similar polymerization process but in the absence of the alkali metal carboxylates. Such a process is disclosed in U.S. Pat. No. 3,919,177.
The wide variety of end use applications which can employ arylene sulfide polymers has been an important factor underlying the need for processes to provide the above-described branched or modified arylene sulfide polymers of reduced melt flow rate as well as processes for producing linear arylene sulfide polymers having a very high melt flow rate.
Another important property of arylene sulfide polymers relates to thermal stability of the polymers. This thermal stability or lack thereof can become very important in processes in which the arylene sulfide polymer is maintained at elevated temperatures for an appreciable length of time. For many applications it is important that the arylene sulfide polymer have a relatively high degree of thermal stability. A very convenient way to ascertain the degree of thermal stability of an arylene sulfide polymer has been to observe the change in the polymer melt flow rate after heating the polymer for specified periods of time and at specified temperatures. The polymer melt flow rate determined after the specified heating period has been designated the "cure rate". The "cure rate" value determined by the method described herein is inversely related to the rate at which the polymer cures. Therefore, polymer having a high "cure rate" value cures slower than polymer having a low "cure rate" value. Thermal stability of a polymer is relative and is determined by comparing one polymer to another. Specifically, the greater the "cure rate" value for polymers of similar initial molecular weight the more thermally stable the polymer is when compared to the other polymers.
A variety of methods have been employed in the art in an attempt to provide improved thermal stability to arylene sulfide polymers. For example, additives of various types have been employed in an attempt to improve the stability of arylene sulfide polymers. However these methods have not always been completely successful and suffer from various disadvantages such as the cost of the added stabilizing additives. Some additives also may deleteriously affect the physical properties of the final arylene sulfide polymer composition which obviously would also be a disadvantage. Thus, other means are desired in order to improve the thermal stability of arylene sulfide polymers particularly those which are characterized as the linear arylene sulfide polymers.
Improvements are always desired in polymerization processes such as those employed in producing arylene sulfide polymers wherein expensive reactants are involved. In such cases it is highly desirable that improvements in polymer yield for the reactor charge be obtained whenever possible so that the cost of producing the arylene sulfide polymers can be correspondingly reduced.
Typical processes for producing arylene sulfide polymers have generally involved the separation of particulate solid polymer containing a certain level of impurities from a liquid phase by means of a filtration step whether in the initial separation prior to any purification steps or during a purification sequence employing a liquid such as water to wash out water soluble impurities from the polymer. Such filtration steps can become time consuming and thus expensive in the process for the production of arylene sulfide polymers. Thus improvements in the filtration rate in the separation of particulate arylene sulfide polymer from a liquid is also highly desirable.
It is therefore an object of this invention to provide a method for producing arylene sulfide polymers which may be linear, branched or modified in a high reactor yield. It is another object of this invention to provide a method for producing arylene sulfide polymers in which the particulate polymer is separated from liquid at a high filtration rate. It is another object of this invention to provide a method for producing arylene sulfide polymers which have a high degree of thermal stability.